In 1996, France detonated a bomb beneath Fangataufa Atoll about 750 miles southeast of Tahiti that had a yield of about 120,000 tons of TNT.

The U.S. Geological Survey said it recorded a magnitude-4.2 seismic event in northeastern North Korea. Asian neighbors also said they registered a seismic event, but only Russia said its monitoring services had detected a nuclear explosion.

No one has reported detecting any radiation.

• Raw Data: North Korean Statement on Alleged Nuke Test

"We know the exact site of the test. The ecological situation is normal, including on Russian territory in Primorye," Defense Minister Sergei Ivanov said, referring to the Russian Far East province that shares a short border with North Korea.

Interfax, citing an unnamed diplomatic source in Moscow, said that the North Korean Foreign Ministry had informed the Russian ambassador in Pyongyang about the test two hours before it was conducted. The report could not immediately be confirmed.